1. Field of the Invention
The present invention relates to an image processing apparatus for performing an appropriate interpolation process on a pixel signal including signals for some of color components in a predetermined color space.
2. Description of the Background Art
Image pickup elements, such as CCDs, CMOSs and the like which are used in digital still cameras and the like, perform photoelectric conversion of light received through color filters, to output pixel signals. Such color filters include RGB color filters, YMCK color filters and the like. Then, in a case of using a single-chip color filter, for one pixel, a pixel signal for one color is outputted. For example, in a case of using the RGB color filter, for one pixel, a pixel signal for one of R (Red) component, G (Green) component and B (Blue) component is outputted.
Thus, since the pixel signal outputted from an image pickup element comprising a single-chip color filter includes a single-color pixel signal per pixel, an interpolation process is performed for pixel signals for other color components. As to such an interpolation process, various algorithms are used.
For example, on a RAW image (an image before pixel interpolation), judgment is made on a correlation between a specified pixel and each surrounding pixel. Then, by using pixels in a direction where the correlation with respect to the specified pixel is high, color interpolation is performed. Thus performed is demosaicing from a RAW image to an RGB image.
In Japanese Patent Application Laid Open Gazette No. 2006-24999, with respect to a specified pixel, correlation values in four directions are calculated and a pixel interpolation process taking the correlation into account is performed.
As discussed above, the interpolation process is a process for estimating image data of the specified pixel from image data of the surrounding pixels. Therefore, when a RAW image having high noise is outputted from an image pickup sensor in a case of high photographic sensitivity or the like, it is not possible to reduce the noise even if the same interpolation process as in a case of low photographic sensitivity is performed. Further, due to high noise, correct judgment can not be always made on the correlation of the image even if it is attempted. Then, if wrong judgment is made on the correlation, there is a possibility of enhancing the noise. On the other hand, when a specified pixel is estimated by uniformly using the average pixel value of the surrounding pixels, it is possible to reduce the noise, but the sense of resolution of the interpolated image is degraded.
In summary, in the background-art technique, in the case of demosaicing for high noise RAW image, if it is intended to achieve good sense of resolution, the noise component is enhanced as its side effect, and if it is intended to remove noise, the sense of resolution is degraded as its side effect.